In recent years, people have become more conscious of the conservation of the global environment, now demanding that the energy consumption should be reduced. The demand for energy saving is growing. People now request that energy saving legislation should be revised and that conformance to standards such as ISO 50001 should be stipulated. Attention is therefore being paid to techniques that can reduce energy consumption in comparatively large facilities, such as buildings and factories.
A large part of the energy consumed in a facility is used in the air-conditioning system. In some buildings, the air-conditioning system is responsible for half the total energy consumed. Hence, if the energy consumed by the air-conditioning system is decreased, energy will be effectively saved in the building. In view of this, various techniques have been proposed, which efficiently operate the system (hereinafter called the “thermal recycling plant”) that recycles thermal generated in the building, thereby raising energy efficiency.
In the existing technique, the control value set to minimize the energy consumption is calculated from the evaluation function based on the specification or operating history of the air-conditioning system. More specifically, the thermal recycling plant is studied as a group of controlled objects, and an optimization based on a modeling technique is performed, thereby calculating the target control values for the respective control objects. In this method, however, the specification or operating states of all components constituting the thermal recycling plant cannot always be taken into consideration. The operating state of the thermal recycling plant depends also on the environmental conditions, such as temperature and humidity. Inevitably, the modeling of the system is limited. Consequently, there may be discrepancy between the control based on calculation and the actual operation of the system.
Assume that, by a calculation for minimize the energy consumption, a temperature (hereinafter referred to as the “preset coolant temperature” is recommended, which is higher than the ordinary value for the coolant temperature optimal at, for example, the output of a cold/heat source apparatus. If the preset temperature is increased, the power to the compressor for cooling the coolant can be reduced, ultimately to decrease the energy consumed in the cold/heat source apparatus. In this respect, the decrease of the preset temperature is theoretically advantageous. However, the cooling capability will decrease, because the temperature difference between the coolant and the any object cooled. As a result, the outdoor air-conditioning unit will have but inadequate dehumidifying capability, and a feedback that lowers the humidity will increase, possibly increasing the energy consumption in some cases.
Used as an index for evaluating the operating state of the thermal recycling plant is the coolant bypass flow rate. The coolant bypass flow rate is an amount in which the coolant flows per unit time flows (or is bypassed) from the outlet port of the chiller recycling thermal from the coolant, to the outlet port of the chiller. This flow rate remains sufficiently high as long as the chiller has sufficient cooling capability. The coolant bypass flow rate, however, lowers if the load (for example, outdoor air-conditioning unit or indoor air-conditioning unit) must operate to achieve cooling. In this case, the standby cold/heat source apparatus is activated before the coolant bypass flow rate falls below a preset value. Thus, the control may be inappropriately performed, in some cases, possibly increasing the energy consumption in the thermal recycling plant in its actual operation, in spite of the target control values calculated for the respective control objects.